The Heat Treat Doctor
Daniel H. Herring | 630-834-3017 | [email protected]
Flowmeter Basics
E
v
veryone
knows what a flowmeter is, and, yet, few of us
rreally understand them the way we should. The sad
rreality is that once flowmeters are installed and operaating we tend to take them for granted. This can often
lead to serious flow errors and potential process or safety issues
that compound themselves over time. Let’s learn more.
What is a flowmeter, really?
A flowmeter is a device used for measuring the flow of gases or
liquids. There are actually two different
ways to measure flow – by volumetric means
(Fig. 2a) and by mass-flow techniques (Fig.
2b). As heat treaters, we are probably more
familiar with the volumetric-flow measurement of gases. The principle involves the
displacement of the gas volume over time.
Atmosphere furnaces, gas generators and
combustion systems typically use these
types of devices. Mass flow involves measuring the weight of a gas, and these are commonly found on vacuum
furnaces that meter in gases for partial-pressure operation.
What types of flowmeters are there?
As heat treaters, we typically focus on the measurement of gases.
Anyone who has seen a nitrogen/methanol system, however, is
aware that liquid methanol must be metered into the furnace.
Flowmeters are also used for measuring liquid flow. In addition to
flowscopes and mass flowmeters, other common types include: or-
ifices, rotameters, positive-displacement meters, electromagnetic
meters, ultrasonic (Doppler-effect) devices, turbine meters, wedge
flow devices, impact meters and turbine meters.
What are the features and advantages of the most common flowmeter types?
Variable-area flowmeters offer:
• Mechanical flow measurement with only a single moving part,
ensuring measurement reliability
• Application versatility and availability of a variety of construction materials, inlet and outlet sizes and types
• Easy installation with generally no straight pipe requirements
• Low pressure drops
• Linear scales, allowing easy flow measurement interpretation
• Electronic output availability, preserving the benefits of mechanical flow measurement
Tapered-tube rotameters offer:
• Low instrument cost (when glass or plastic metering tube is used)
• Accuracy at very low flow rates
Slotted-cylinder flowmeters offer:
• A flow range of 25:1 since flow-measurement accuracy is determined by the precision of the slot manufacturing operation
• Instrument specifications can be changed by field replacement
of the slotted tube and float, without having to repipe the flowmeter body
• Ability to handle high flows and pressures
• Improved immunity to the effects of pulsating flows, with no
minimum back-pressure
Limitations common to both tapered-tube and slotted-cylinder
variable-area flowmeters are that vertical mounting is required
and that they contain moving parts.
The user should also be aware that the accuracy of mass flowmeters and mass-flow controllers is determined by two factors: flow
calibration and repeatability. Proper instrument calibration ensures
starting-point accuracy. Repeatability is the measure of continuous
performance-to-specification over the lifetime of the device. Most
mass flowmeters and mass-flow controllers have an accuracy of ±1%
of full scale and a repeatability of ±0.25% of full scale.
Fig. 1. Typical flow control panel (multiple-zone continuous furnace)
18 April 2011 - IndustrialHeating.com
Is it easier to control a gas or a liquid?
Interestingly, liquids are easier to measure and control because
of their small compressibility. For most volumetric-flow applications, the incoming pressure in liquid systems does not need to be
closely controlled. By their very nature, liquids can be captured
•
•
rial H
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easily and measured to a high degree of accuracy. Gases on the other hand, because
of their compressibility, require more complex sensing and control methods.
What is the accuracy range of a gas
flowmeter?
When measuring gas flows in heat-treating applications there is an important
distinction between the operating range
of a flowmeter and the design range when
purchasing a new meter. Plan to operate a
flowmeter in a range not below 25% and
not above 90% of the flowscope’s scale capacity. In other words, if your flowmeter is
rated for 0-2,000 cubic feet per hour (cfh),
you can be assured of accuracy when the
flow is between 500 cfh and 1,800 cfh.
Flows outside these limits are not considered accurate.
A good “rule of thumb” for sizing a
flowmeter is to purchase your meter “in
the middle third.” The flowmeter should
be chosen so that the actual flow will be
no less than 1/3 and no higher than 2/3
of the scale you select. This gives you the
ability in actual operation to compensate
for unexpected changes in flow requirements that may occur. Often over the life
of a heat-treating furnace, process requirements and operating conditions change –
sometimes dramatically – and you want
your
to remain accurate.
y
ggas measurement
su
Should I have my flowmeters
recalibrated?
If a change of operating conditions is permanent, such as the desire to constantly
operate at a different pressure, then recalibration of the flow measurement device
is strongly recommended. As a rule, flowmeters used in heat-treating applications
are designed for a maximum temperature
of 150˚F (66˚C) and an operating pressure
up to 50 psig (345 kPa). However, application-specific flowmeters have maximum
operating pressures outside these ranges.
What affects my gas measurements?
If knowing the proper flowrate is important to you, be aware that a change in
temperature, pressure or specific gravity of
the gas from that for which the meter was
calibrated will cause a serious error in the
indicated scale reading. (These topics are
covered in detail in the online portion of
this column.) It is quite common in a heattreat shop to find flowmeters operating at
different pressures and temperatures than
they have been calibrated for.
Do I need to maintain my flow
devices?
All flowmeters eventually require maintenance. It is a sad truth that some units
require more maintenance than others,
so this factor should be considered when
Electronics
1
Sensor
tube
Sensor
seals
Bypass
Value
actuator
Do I really need to learn about my
flowmeters to be in control, stay
in control, operate safely and keep
the cost of my operation as low as
possible?
Simply stated, yes. Hopefully, this discussion has helped reinforce this idea. Now go
out today and check your flow devices! IH
This column concludes online.
Valve seal
Valve orifice
Fig. 2. Flowmeter types (a) Endothermic gas FurnaceMeter (Courtesy of Atmosphere Engineering Company); (b) Mass-flow controller (Courtesy of MKS Instruments)
20 April 2011 - IndustrialHeating.com
a unit is selected. However, in most heattreating operations the equipment manufacturer has already made that choice for
you, so understanding what maintenance
is required and when it should be performed is of paramount importance.
Flowmeters have moving parts and
require internal inspection, especially if
the fluid is dirty or viscous. For example,
in furnaces using endothermic gas, the
flowmeters often become contaminated
with soot (carbon) and must be cleaned
by CAREFULLY disassembling the flowmeter and cleaning all internal moving
parts as well as replacing the dirty fluid
in the flowmeter tube. Caution: This involves isolating the flowmeter, or performing maintenance when the unit is shut
down, and must be done in a safe manner
as many of the gases involved are asphyxiants as well as being flammable, toxic and
possibly life-threatening.
Remember also that electromagnetic
flowmeters and all flow measurement
devices that use secondary instruments
such as pressure sensors to actuate a control valve or send a signal to a remote
source must be periodically inspected,
calibrated, repaired and/or replaced. Improper location of the flowmeter itself,
the secondary sensor or readout devices
can result in measurement errors and
hidden costs.
There was too much good information to contain in this column.
Find the rest with this Tag, or go to
www.industrialheating.com/meter
References:
1. “North American Combustion Handbook,
2nd Edition,” North American Manufacturing Company, Cleveland, OH.
2. Braziunas, Vytas and Daniel H. Herring, “A
Flowmeter Primer,” Heat Treating Progress,
March/April 2004.
3. Mr. Eric Jossart, Atmosphere Engineering
Company, private correspondence.
On-Line Content
What do I need to consider if I want
to use a flowmeter for one gas, but
it is calibrated for another?
One of the most common problems seen
in heat-treat shops is that operating personnel and supervisors are unaware of the
consequences of a flowmeter that has been
calibrated for use with one gas while having another gas flowing through it. When
switching gases, the change in specific
gravity of the two gases is the principle
factor that must be taken into account.
Specific gravity is the ratio of the density
of the gas under consideration to the density of dry air (at standard temperature and
pressure, 77˚F (25˚C) and 14.7 psi).
Table 1 provides a quick conversion
chart. To calculate the actual flow rate of
gas being metered, multiply the indicated
scale reading of the flowmeter by the factor
shown in the table.
Equation (1) allows us to calculate the
actual flow when a change in specific gravity occurs.
(1)
Where: Fa = actual flow
Fi = flow indicated scale reading
SG2 = specific gravity of gas to be
used in the flowmeter
SG1 = nameplate (calibrated)
specific gravity
How do I compensate for changes in
pressure and temperature?
A change in temperature and/or pressure
of the gas from for which the meter was
calibrated will cause a serious error in the
indicated scale reading. However, there is
an easy way to calculate the effect of these
changes.
Temperature Compensation
Equation (2) allows us to calculate the
actual flow when a change in temperature
occurs.
(2)
Where: Fa = actual flow
Fi = flow indicated scale reading
T1 = nameplate (calibrated)
temperature + 460 (English
system) or nameplate (calibrated)
temperature + 273
(metric system)
T2 = new temperature + 460
(English system) or new
temperature + 273 (metric system)
A summary for some common values is
shown in Table 2.
Table 2. Temperature conversion factors
Temperature, ˚F (˚C)
Scale multiplier
50
(10.0)
1.02
60
(15.6)
1.01
70* (21.0)
1.00
80
(26.7)
.99
90
(32.2)
.98
100 (37.8)
.97
110 (43.3)
.965
120 (48.9)
.96
130 (54.4)
.95
140 (60.0)
.94
150 (65.6)
.93
* Flowmeter calibrated for 70˚F (21˚C)
Table 3. Pressure conversion factors
Table 1. Specific-gravity conversion factors (Courtesy of Waukee Engineering Company)
Pressure, psi (kPa)
Scale multiplier
0.5* (3.4)
1.00
1
(6.9)
1.03
2
(13.8)
1.06
3
(20.7)
1.10
4
(27.6)
1.13
5
(34.5)
1.16
10
(69.0)
1.30
15
(103.4)
1.42
20
(137.9)
1.53
30
(206.9)
1.75
40
(275.8)
1.93
50
(344.8)
2.10
*Flowmeters are typically calibrated for 0.5 psi (3.4 kPa)
Pressure Compensation
Equation (3) allows us to calculate the actual flow when a change
in pressure occurs.
(3)
where: Fa = actual flow
Fi = flow indicated scale reading
P2 = new inlet pressure + 14.7 (English system) or new
inlet pressure + 101.3 kPa (metric system)
P1 = nameplate (calibrated) inlet pressure + 14.7(English
system) or nameplate (calibrated) inlet pressure +
101.3 kPa (metric system)
A summary for some common values is shown in Table 3.
If it is necessary to compensate for both temperature and pressure,
Equation (4) should be used.
(4)